An optical fingerprint sensor is known, which uses a prism and a photoelectric converter. The photoelectric converter converts incident light rays reflected by ridges and valleys of finger surface that is in contact with a surface of the prism.
A fingerprint sensor using a pressure sensitive sheet is known. Arranged in a matrix form on the pressure sensitive sheet are electrodes, Distribution of electrical resistance over the whole area of the sheet is altered by ridges and valleys of finger surface that is in contact with a surface of the sheet.
A fingerprint sensor is known, which uses capacitance formed between a pair of electrodes on surface of an insulator. Distribution of capacitance varies in response to ridges and valleys of finger surface that is contact with surface of the sensor.
JP-A 5-61965 discloses a fingerprint sensor including a number of sensor elements arranged in a matrix form on a semiconductor substrate. Each sensor element includes a piezoelectric thin-film, a switching field-effect transistor (FET), and a FET for impedance conversion and/or output amplification.
JP-A 61-222178 discloses a pressure sensor using a FET. According to this known pressure sensor, an elastic insulating layer is deposited on a gate insulating film that is deposited on a channel region of the FET between a source region and a drain region, and a gate electrode is formed on the elastic insulating layer. When the gate electrode is subjected to mechanical stress, the pressure sensor detects the stress as variation in the drain current. The drain current is proportional to the magnitude of stress applied to the gate electrode. This is because the drain current varies with electric field applied to the channel region, and the strength of this electric field varies with variation in distance between the gate electrode and the gate insulating film. To increase a ratio of the distance to the stress, the elastic insulating layer is formed with an opening above the channel region.
In the prior art, the optical fingerprint sensor requires a source of light and a photoelectric converter such as a CCD camera, resulting in a bulky and expensive apparatus.
The fingerprint sensor using a pressure sensitive sheet is difficult to fabricate due mainly to electrically conductive rubber is used to form the pressure sensitive sheet. Thus, quality and reliability control is difficult.
The fingerprint sensor, which uses capacitance formed between a pair of electrodes on surface of an insulator, is susceptible to occurrence of current leak due to stain on the sensor surface.
The fingerprint sensor, which is disclosed by JP-A 5-61965, is susceptible to a drop in its output because electric charge decreases in response to current leak within the piezoelectric thin film.
The pressure sensor, which is disclosed by JP-A 61-222178, uses a FET. As is well known, the FETs exhibits temperature dependent characteristics. Thus, a sensor using FET alone has a potential problem that output of the FET must be corrected to compensate for a deviation due to change in temperature. The elastic insulating layer deforms upon application of a force to be measured is applied to the gate electrode. The ratio of vertical distance between the gate electrode and the gate insulating film to the magnitude of force applied cannot be improved to a satisfactorily high level. This is because the deformation of the elastic insulating layer in horizontal directions is limited and cannot be expected to such a degree as to provide a desired increase in the vertical deformation. Thus, this known pressure sensor cannot be employed as a sensing element of a fingerprint sensor.
The present invention provides a tactile sensor, which may be employed as a sensing element in a fingerprint sensor.
An object of the present invention is to provide a compact and thin fingerprint sensor, which can detect fingerprint image of a finger surface with high fidelity and stability over wide range of varying temperatures.